CHAPTER 2

GEOLOGY

Page No.

2.1 Introduction 28

2.1.1 Selection of the Study Area 28

2.2 Geographical Location 30

2.2.1 Topography 30

2.3 Geology 30

2.3.1 Granites and Gneisses 31

2.3.2 Porphyritic Granite 35

2.3.3 Pegmatite, Aplites and Quartz Veins 35

2.3.4 Gabbros 36

2.3.5 Epidiorite 36

2.4 Lithology 37

2.5 Stratigraphy 38

2.6 Structural Features 39

2.7 Rift, Grain and Hardway 40

2.8 Sheeting 40

2.8.1 Joints 40

2.8.2 Fractures 41

2.8.3 Lineaments and Faults 41

2.8.4 Strike and Dip 42

2.9 Regional Setting 42

3.0 Dolerite Dykes 44

3.1 References 46

28

CHAPTER 2

GEOLOGY

2.1 Introduction

The Indian Peninsular shield is infested by a large number of dyke swarms.

The geological investigations of an area form an important basis for various

geophysical surveys to have a comprehensive picture under study. A classification of

hard consolidated rocks like granites and dykes according to their mineral

composition. Granites are also characterized by horizontal joints (Sheeting) which are

parallel to the surface. Granites are deeply weathered either by mechanical or

chemical processes resulting in formation of a thick covered mantle of residual

material commonly designated as disintegrated rock, which do not have primary

porosity and permeability and hence are poor water bearing formations. Joints,

fractures and other structural features can be developed.

2.1.1 Selection of the Study Area

The study area, which forms part of Greater Hyderabad and Ranga Reddy

District covering an area of about 2500 s.q. km is situated at about 50km north of

Hyderabad, Andhra Pradesh, India and is well connected by road. The area is covered

mainly by granites of Archaean age which are highly weathered and fractured. A large

number of out crops are available to map the structures and fractures. Number of

intrusive like dolerite dykes, Pegmatite veins and Quartz reefs/veins are common in

the area. Detailed geological and hydrogeological investigations were carried out with

an objective to demarcate different rock outcrops, exposures in the study area

measurement of structural elements and general assessment of the hydrogeological

conditions prevailing in the area. The map showing the study area is presented in Fig.

2.1

29

Fig 2.1 Location map of the Study Area

30

2.2 Geographical Location

The Mandals of Ibrahimpatnam, Keesara, Ghatkesar and Maheshwaram in

Greater Hyderabad and Ranga Reddy district have been selected for the present study.

These areas are continuously extended towards SSE, ESE, NE, SE, East ENE of

Hyderabad city. The study area forms part of Quadrangle Map or Topo sheet No 56k.

Hyderabad being the state capital and is also the headquarter of Ranga Reddy district,

is very well connected with all other state capitals by air, rail and road. Two National

high ways, namely NH-7 and NH-9 pass through these districts.

Ibrahimpatnam, Keesra, Ghatkesar and Maheshwaram Mandals are very well

connected by road. Ibrahimpatnam is on main road of Nagarjuna Sagar ring road and

SSE of Hyderabad road. Keesara and Ghatkesar are situated on National high way

No. 9 towards east south east. Maheshwaram is situated on National high way No.9

towards southwest. Maheshwarm is the near Shamshabad Airport, which was

connected with the number of buses from Greater Hyderabad and Ranga Reddy

District. Fair weather and network of macadamized roads and a few pedal path and

foot tracks connecting the local villages provide accessibility to the interiors of the

area.

2.2.1 Topography

Topographically the study area is generally undulating or flat tracts characterize

with isolated relief hillocks and hill ranges with intervening streams. Dolerite dykes,

quartz reefs have developed their own prominence in the investigated area.

The weathering plays major role in the development of the topographical

features. Joints plains have added advantage during the weathering process on account

of which smooth conical tors, rounded hills have developed into characteristic land

forms in this area. The long hillock ranges are in the western part of the study area.

2.3 Geology

The Indian Peninsular shield is infested by a large number of igneous dykes

which constitute swarms in a few localities. These dykes which may be of different

ages strike in different directions, NE-SW, ENE-WSW or NW-SE and are seen as

31

persistent linear ridges extending for several kilometers with width varying between 3

to 60 meters.

2.3.1 Granites and Gneisses

The granitic rocks of the world are the subject of intense investigation

regarding their origin and as read as put it “There are granites and granites”.

The study of Precambrian shield granites present many problems as they are

subjected to many phases of deformation, metamorphism and igneous activity. The

granites of Hyderabad belong to the Precambrian shield type and consist of many rock

types from syenites to Alaskites.

Precambrian rocks, such as Granite, Adamellite, Tonalite, Amphibolites,

Hornblende biotite schist occupy a major part of the area. These formations were

subjected to tectonic and green schist facies metamorphism. Except for portion in the

western part of the district to understand its geological significance most of the area is

occupied by granites. Based on mineral composition they were further classified as

alkali feldspar granite, Magmatite granite gneiss, Adamellite, Granodiorite, Tonalite

and Trondjimite.

Granites and gneissic complexes, occupy vast expenses of Peninsular India.

Granitic rocks of the Hyderabad form part of the Peninsular Gneissic Complex. Pink

and Grey granites are the major rock types occurring in and around Greater

Hyderabad. Dolerite dykes, basic enclaves, aplites, pegmatite, quartz veins,

migmatites and alaskite traverse these granites. At places alaskites and leucogranites

are emplaced within the pink and grey granites (Krishna, 2004). Mineralogy, textures,

structure and major intrusive of these areas are

The major units are:

1). Grey series: coarse porphyritic grey granites, biotite granites and pyroxene

granite.

2). Pink series: coarse porphyritic pink granite, fine to medium grained pink granite,

coarse porphyrite pink alaskite, fine grained pink alaskite, white alasklite.

3). Dykes and veins: Dolerite dykes (both fine and coarse grained)

Quartz veins and epidote veins

32

4). Enclaves: Fine grained

5). Migmatitic zone: Inter mingling zones of pink and grey granites.

Investigated study area. Fig. 2.2

Fig. 2.2 Geology Map of the Study Area

and it covers the cluster of Mandals i.e. Ibrahimpatnam, Keesara, Ghatkesar and

Maheshwaram Greater Hyderabad and Ranga Reddy district. Almost all the Mandals

are covered with Peninsular granites. The Granites of Hyderabad have been

extensively studied during last three decades by a number of researchers,

(Balakrishna, 1964; Janardhan Rao, 1965 and Sitaramayya, 1968). The rock types

covered in the area are classified as grey and pink granites and they are intruded by

younger formation i.e. dolerite dyke, quartz, epidote and Pegmatite veins.

33

Raja (1959) attempted systematic studies on the petrography of the granites

occurring in and around the twin cities of Hyderabad & Secunderabad. According to

him, soda metasomatism of pre-existing hornblende – biotite schist gave rise to grey

granites. The parental rock occurs as enclaves and the pink granites were derived from

the granites by potash metasomatism. Rao (1963) suggested that there might not be

any age difference between grey granites and pink granites. He opined that the

difference in the colour of the two granites could be due to the difference is the

parental material, which was subjected to process of granitization. Rao (1965) opined

that both grey and pink granites are metasomatic in origin. The grey granites were

formed by granitization of parental metasediments like quartz mica schists and related

rocks. The pink granites were supposed to have formed along shear zones in grey

granites by potash metasomatism.

There are several researchers who have expressed different opinions regarding

their origin. Geological Survey of India, Hyderabad has concluded that the granites of

Hyderabad are of magmatic origin. Balakrishna, (1964) studied in detail the Petrology

and Petrogenesis of these granites. They came to the opinion that the granites have

formed due to metasomatism of the country rock.

According to (Balakrishna, 1964) granitc rocks predominate the peninsular

complex and are represented in the city of Hyderabad. He studied the granitic rocks of

Hyderabad with special reference to tectonics, petrography and textural features. He

noticed two types of granites, pink and grey, and said that it has been found difficult

to demarcate them separately. On the basis of his studies he concluded that the pink

could be a derivative from the grey series of granites and gneisses. He also concluded

that the pyroxene bearing granites seem to have all characteristics of the conventional

charnokites and they appear to be a sort of basis front formed due to late stage of

potash feldspar. He further said that, the pink granites are found to be rich in perthite

and myrmekitic structure and are seen in all types of granites particularly in hybrid or

magmatic zone. He also studied the elastic constants pointed out that the pink granites

are younger than the grey granites.

Janardhan Rao and Sitaramayya, (1968) has extensively studied the

Geological features like structures, the petrology and geochemistry of granites of

34

Ghatkesar area near Hyderabad and opined that the pink alakites and basic dykes are

of anatectic origin. Janardhan Rao, (1965) has studied the origin of Hyderabad

granites.

Janardhan rao, (1965) is of opinion that these granites could ever passed

through a magmatic stage. The parallelism of the structural elements of the granites

with the Dharwar meta-sediments, the textural characters between the major mineral

constituents, the enormous heterogeneity in the composition of the granites in a very

small area, all go against magmatic origin. He also opines that all the granitic rocks

and their associated pegmatite and quartz reefs are metasomatic origin and also says

pink granites are associated with shear zones in grey granites. Age of this granite,

2490± 115 m.y. is very close to closepet granites (Crawford, 1969). Studied

peninsular granites and gneisses and the ages of many of the granites in peninsula are

similar.

According to (Baleequddin Hussain, 1968), Grey granites are due to the soda

metamorphism. The pink granites were formed during the stage of potash

metamorphim. The pink granites follow certain shear zones in the masses of grey

granites. He also indicated parallelism in the structural elements like foliation,

lineation, basic enclaves and dykes and the NNW-SSE trend is parallel to the

Dharwarian trend. He is of the opinion that the granites of Hyderabad are of

metasomatic origin derived from a parent rock of hornblende-biotite- schists type and

suggest that the granites might have passed through a magmatic stage. Work done by

(Raghava Rao, 1974), and (Satyanarayana, 1983) in different part of Hyderabad area,

together concluded that the granites are formed due to metasomatic processes.

These rock types in this region is characteristic of the Archaean granite terrain.

Due to similar rock type mineralogy of both are described together. The rocks of the

areas in grain size from medium to coarse grained. Feldspar is the predominant

mineral and occurs as phenocrysts. These rocks are essentially composed of quartz,

potash feldspar, plagioclase hornblend and biotite and accessory minerals as

magmatite, epidote and sphene. Few specks of pyrite and chalcopyrite are noticed in

the rock samples of the area.

Gnaneshwar (1986) concluded in his litho geochemical surveys of Hyderabad

granites that porphyritic granites, white alaskite and pink alaskite are potentially the

35

ore-bearing rock types. The porphyritic granites contain higher amount of U, Th and

Mo. In the vicinity of pink alaskite and these elements were mobilized and emplaced

in fracture systems at higher structural levels.

The geological Survey of India (2006) have investigated this area and

suggested that these granite rocks are of magmatic origin and pink series are intrusive

into grey series. A very large area of the investigation is underlain by Peninsular

gneissic complex of Archaeans age, consisting of pink and grey granites, granitic,

magmatites, Pegmatites, quartz veins and dolerite dykes, occurring in the forms of

domes, scaprs, massive columnar blocks and ‘tor’scattered over a partly undulating

country.

2.3.2 Porphyritic Granite

These are the dominant rock type of pink and grey colour in this area. Grey

porphyritic granite is predominating in the area of investigation. They are mainly

comprised of microcline perthite, plagioclase and biotite. At some places hornblend

occurs in addition to biotite. Magmatite, sphene, zircon and pyrite are the important

accessories in these granites. The variation in texture and composition is attributed

due to the assimilation of the pre-exisiting rocks of porphyritic granite. Band of

composite gneiss occurring at places in the porphyritic granite might have formed

earlier than the porphyritic granite though injection of granitic fluids along the

foliation planes of the pre-existing schist and reaction with them.

The structure of the porphyritic granite is generally massive, but where it is

composite in character it shows foliation. The foliation is derived from the pre-

exisiting schist’s which are granitised. This has a NW-SE strike and vertical dips. The

porphyritic granite is traversed by many joints. They generally belong to NE-SW and

NW-SE set, and the dip of the joints is vertical. Apart from these are sheet joints,

curved and low dipping in the porphyritic granite.

2.3.3 Pegmatite, Aplites and Quartz Veins

Porphyritic granites are traversed by numerous veins of pegmatites with

different trends. The common trends are NE-SW, NNE-SSW, NNW-SSE and NW-

SE. They are found in white, grey, pink and milk white colours. Feldspar is generally

36

microcline perthite. Apart from feldspar and quartz the other constituents are biotite,

magnetite, muscovite, sphene and zircon. Among the sulphides are pyrite,

chalcopyrite and molybdinite. The molybdinite bearing pegmatites traversing the

porphyritic granites are older than the medium grained granites (Kittu, 1968). Aplites

which are exposed in a few places occur either as narrow veins or small lenses.

The trend of the quartz veins is NE-SW. These veins are found in the

porphyritic granite and at some places quartz veins passes into pegmatites. Generally

they are entirely made up of quartz.

The pegmatites contain mostly feldspar and quartz and quartz veins traversing

these granites occasionally contain pyrite, chalcopyrite and malachite. These are

minor intrusive & traversing the host rocks granites. They are a few centimeters to

few meters wide and run over a few hundreds of meters of distance.

2.3.4 Gabbros

Gabbro is a coarse-grained, dark-colored, intrusive igneous rock. It is usually

black or dark green in color and composed mainly of the minerals plagioclase and

augite. It is the most abundant rock in the deep oceanic crust.

Gabbro is composed mainly of calcium-rich plagioclase feldspar and

clinopyroxene (augite). Minor amounts of olivine and orthopyroxene might also be

present in the rock.

This mineral composition usually gives gabbro a black to very dark green

color. A minor amount of light-colored mineral grains may be present. Unlike many

other igneous rocks, gabbro usually contains very little quartz

2.3.5 Epidiorite

Epidiorite dykes are noticed in this area. They are mostly in NE-SW direction they are

generally composed of amphibolites and andesine. Hornblende generally occurs as

pseudo-morph of pyroxene. The feldspar is very much altered.

Flourite, Epidote, Calcite, Feldspar, Quartz and Jaspar veins are travelling the

porphyritic granites, numerous veins of epidote, calcite, feldspar; quartz and jasper are

37

noticed with less than a centimeter in width. In few cases pyrite, chalcopyrite, biotite

and sphence are present. Epidote veins traverse the epidiorite, pegmatites and quartz

veins. The veins in general have NE-SW strike and steep dips.

The structure of the porphyritic granite is generally massive, but where it is

composite in character it shows foliation. The foliation is derived from the pre-

exisiting schist’s which are granitised. This has a NW-SE strike and vertical dips. The

porphyritic granite is traversed by many joints. They generally belong to NE-SW and

NW-SE set, and the dip of the joints is vertical. Apart from these are sheet joints,

curved and low dipping in the porphyritic granite.

2.4 Lithology

Peninsular rocks, a major part of the area are covered by Pink and Grey

granites. The variations in the granites, the country rocks, are quite apparent through

the two major types, pink and grey are being recognized, each one of them show

several minor variations. These variations are mainly due to mineralogical

composition textural difference, coarse to medium grained, fabric, and enclosers. The

enclosers may be schistose pockets which are referred xenoliths. These xenoliths are

considered to be remanents of pre-existing rock masses. Besides xenoliths, aplites,

pegmatites epidote veins and quartz veins are also responsible for lithological

variations. On a large scale the enclosers are formed from a dolerite cutting across the

lineation and foliation of the granite. The size of dolerite dykes both in width and

length is variable. Some of the dykes traverse this granite for kilometers together with

definite trends. Some times these dykes show enclave pattern indicating that they are

followed the joints plains. Branching, piching and swelling nature is a characteristic

feature of some of these dykes.

All these variations exert considerable influence on weathering and

consequent evolution of land forms. The major contrast is found that the grey granites

are not generally prone to weathering while pink masses are less resistant.

Consequently one finds large terrain being covered with black soils. Most probably

some of these are derived from dolerite dykes. As such the importance of lithology is

to be recognized in the development of soils and evolution of land form which has an

explicit control on occurrence.

38

Apart from these rocks the younger intrusive of dolerite dykes, quartz,

Pegmatite veins, Basalts are present. Geologically the area consists of Predominantly

Peninsular suite of rocks. Based on mineral composition, they were further classified

as alkali feldspar granite, migmatite granite gneiss, adamellite, granodiorite, tonalite

and trondjhemite.

A few patches of amphibolites and pyroxene granulites are also seen around

the Hyderabad. All these rock formations are traversed by NE-SE and N-S trending

dolerite dykes. The area experienced volcanism at the time of eruption of the Deccan

Traps. The Deccan Traps and intertrapean beds are seen in the western part of the

study area.

2.5 Stratigraphy

The investigated area is occupied mainly granite rocks of Archaean age

comprising grey and pink with medium to coarse grained texture cover a major

portion of the study area. In palaeomagnetic study, Proterozoic to Archaean age acid

intrusive pegmatite, quartz reef/veins and migmatites occur in the study area. The

granites were also intruded by Paleocene to Cretaceous age dolerite dykes and basalts

and represented in geology map of the area.

The stratigraphic succession is given below:

Basic dykes

Basic enclaves/Quartz reefs

Peninsular gneissic complex

Dykes Porphyritic type

Fine grained type

Leucocratic type

Mesocratic type

Melanocratic type

Pink granites

Fine grained

Medium grained

39

Grey granites

Coarse grained

Gneissic type

2.6 Structural Features

The investigated area is mainly occupied with hard rock formations. In such

cases which are devoid of any primary porosity, secondary structures like faults,

joints, cleavage fractures, lineation, and foliation serve as channel ways for the

circulation of palaeomagnetism and dykes. Hence, the study of these features is

essential for proper understanding of the occurrence and behavior of dykes in these

formations.

First, many exposed granitic bodies display three sets of joints disposed at

right angles to each other or mutually. These are together referred to as orthogonal

system and they effectively divided the mass Granites. Even though granites are quite

massive and impermeable in their original unmodified state; they are generally

rendered pervious with the development of joints and fractures.

In the investigated area the regional structures and fracture pattern control the

topography. The trend of the main structural features is NNW-SSE and the alignment

of bed rock exposures in the study area mainly follows the same trend.

National Remote Sensing Agency (1978) employed the satellite imaginary

techniques to unravel the lineament pattern over the granite-gneiss terrain of Andhra

Pradesh. Their studies indicate “lineaments” in almost all directions of A.P. However,

the intensity and direction vary from area to area.

The most important directions are as follows:

a). NNE to SSW

b). WNW to ESE

c). ENE to WSW

d). NW to SE and

e). NE to SW

The two trends NNW-SSE and E-W are best displayed in the study area

40

The granitic rocks of the area possess certain characteristic structures in great

perfection.

2.7 Rift, Grain and Hardway

These planes approximately at right angles to one another along which granite

falls most easily under tension. The rift is due to the peculiar property of quartz and is

approximately horizontal in most of the granites in the investigated area. The grain is

usually in the direction to the rift and may be determined by tectonic cracks or other

features. The joints trend to be more consistent in altitudes than the cleavage. When

there are systems, they are equally spaced. The fractures planes give rise to a structure

of cuboidal blocks. However in many instances the jointing is much less regular. This

features is commonly seen in most of the outcrop areas.

2.8 Sheeting

This is the most important structure, particularly well developed at shallow

depth and usually observed in the outcrops of Ibrahimpatnam, Ghatkesar and

Maheshwaram and other areas. Sheeting is kind of horizontal feature. Though it

shows curved aspect on domal hills and resembles exfoliation shells. It will be nearly

horizontal at ground level. All types of granites are sheeted.

2.8.1 Joints

The concept of joints are studied & explained by different researchers in

several types. The simplest of all classification is based on the criteria of degree of

openness (Babuskin, 1972). Joints can be classified as 1). Hidden joints, these are not

seen by naked eye, 2). Closed joints, these are seen by naked eye, 3). Open joints with

the walls clearly separated.

According to (Balk, 1937) classified joints into 1). Cross joints, 2).

Longitudinal joints and 3). Diagonal joints.

Based on the origin (Babuskin, 1972) classified the joints 1). Tectonic 2). Weathered

3). Separater 4). Drained 5). Gravitational.

41

The joints directions can be explained & obtained by various surface and as

sub-surface geophysical methods. Among the surface geophysical methods vertical

electrical resistivity method is used for delineating major joint directions and fractures

occurring up to a depth.

2.8.2 Fractures

These may be called closely spaced joints developed due to tectonic forces.

Intense shearing often produces potential fracture zone which would constitute highly

productive aquifers.

2.8.3 Lineaments and Faults

The area reveals several lineaments and faults identified running different

strikes. The lineaments and faults in the study area show NE-SW, NW-SE and N-S

trend is mainly associated with these lineaments and faults. Fig.2.3

Fig. 2.3 Structural Features Map of Study Area

42

The outcrops of the rock units have been demarcated by trekking the area.

Rocky outcrops of granites in the study area are well exposed in the central, north

eastern and south western part. Detached boulders of pink and grey granite are seen

all over the place due to intense fracturing.

2.8.4 Strike and Dip

Strike and dip refer to the orientation or attitude of a geologic feature. The

strike line of a bed, fault or other planar feature is a line representing the intersection

of that feature with a horizontal plane. Strike (or strike angle) can be given as either a

quadrant compass bearing the strike line (N25˚E for example) or in terms of east or

west of true north or south.

The dip gives the steepest angle of descent of a titled bed or feature relative to

a horizontal plane and is given by the number with degrees (0˚-90˚) as well as a letter

(NS-EW) with direction in which the bed is dipping.

2.9 Regional Setting

These areas are covered under the investigations come under hard rock

terrains as far as palaeomagnetic study investigations are concerned. The chief rock

types are plutonic complex consisting of peninsular granites and gneiss of the

Archaean age. Which form a part of the vast Precambrian Peninsular shield.

Subsequently Balakrishna, (1964) suggested that the pink granite could be

metasomatic derivatives of the grey series of granite and gneiss. Janardhan Rao,

(1965) has argued against magmatic origin of these granites based on the parallelism

of the structural elements in the granites with Dharwarian metasediments and certain

mineralogical and textured relationships.

The different rock types occurring in this area have been broadly classified

(Sitaramayya, 1968) into three groups. First two groups belong to granites while the

third group comprises the dolerite dykes. Among the first two groups, grey and pink

series have been identified.

43

The grey granites are fine to medium grained, dark coloured and appear

homogeneous from a distance. However, on close observation, they show small

clusters of mafic minerals, pyroxene and biotite. These clusters are arranged in

parallel planes and constitute the lineation causing pronounced gneissic banding. The

quartz grains in these rocks have light brown colour. They have greasy appearance.

Feldspar crystals are light brown colour and plagioclase grains are semi opheic with

white or light or light green colour.

Sometimes these rocks show patches of porphyroblasts of potash feldspar. The

gneissic bonding shows dominant trend of N50˚W. These rocks are traversed by

aplitic veins and dykes of width ranging from a few centimeters to 100 meters. Many

epidote veins come across these granites.

The pink granites are usually fine to medium grained but coarse grained

porphyritic varieties are not uncommon. These are characterized by a very low mafic

mineral content and have pink colour. The minerals are quartz and potash feldspars.

Minor amounts of plagioclase, biotite, magmatite and zircon are present. Usually

these rocks occur as ridges sometimes rising to a high top, more than 80m. They are

having consistent structural trends, the important trend is N 30˚-40˚W followed by the

N-S N60˚-70˚E. These rocks sometimes are characterized by pronounced foliation

due to the parallel arrangement of quartz grains.

These rocks exhibit a characteristic weathering which is controlled by closely

placed joints in more than one set. Many narrow quartz veins also traverse these

rocks. These rocks are to be considered younger to the grey series of rocks but to

dolerite dykes. It is considered that the emplacement of basic dykes followed weak

structural planes in the country rocks.

Both the pink and grey granites are traversed by quartz and pegmatite veins.

Intrusive bodies were more in to pink granites than in grey granites. The demarcation

between pink and grey granites is difficult. In pink granite itself there are several

distinct types. Some of them are brick red in colour, others are bright pink and still

others show all shades of pink red and reddish brown.

44

3.0 Dolerite Dykes

Dolerite is coarse grained variety of basalt, if present in the dorm of a dyke

(discordant igneous body), it is called a dolerite dyke. Dolerite is the medium grained,

intrusive, equivalent of a basalt (link to basalts). It usually occurs as dykes. Being

intruded into country rocks at shallow levels, the magma has more time to cool than if

extruded. Dolerites may be porphyritic, containing phenocrysts of plagioclase, olivine

and even quartz (an olivine analcite dolerite is known as a crinanite).

Dolerite dykes, like many other magmatic intrusions, develop by rapid

hydraulic fracturing via the propagation of a fluid-filled open fissure, resulting in a

massive magmatic intrusion with a neat and transgressive contact with the country

rock. This fracturing mechanism is in contrast to the slow mode of hydraulic

fracturing responsible for breccia-intrusions (kimberlite). For the intrusion to develop

the magma pressure at the tip of the fissure must overcome the tensile strength of the

surrounding rock. Dykes can develop vertically upwards or laterally along-strike over

very long distances, as long as the magma pressure at the tip of the fissure is

maintained. The intrusion of dolerite and basaltic dykes are therefore never

accompanied by brecciation, deformation or shearing of the host-rock, at least during

their propagation.

The main gabbroic body is intruded by a member of dolerite dykes running

North- South. It is composed of clinopyroxene, Plagioclase, Hornblende,

Orthopyroxene and Ilmenite. Hornblend occurs as large Poikilitic grain and

constitutes around 60% of the rock. Both gabbro and associated dolerite dykes

(Aditimaddal et al., 2012). The Shillong Plateau occupies a unique position in the

Precambrian history of the Indian plate both geodynamically and seismically. The

Precambrian framework of Meghalaya and tectonics of NE India were given by

(Mazumber, 1986) and (Acharya, 2005). In the central and northwestern shilling

plateau a number of Mesozoic dolerite/basalt and ultra basics have intruded the

basement rocks in NW-SE. Greenstone are also exposed in NE-SW direction within

the gneisses (Nivarani Devi and Sarma, 2006). Palaeomagnetic and geochemical data

of the Proterozoic Khasi greenstone dykes to understand the evolutionary history of

the Shillong basin vis-à-vis the Shillong plateau (Mallikharjuna Rao et al., 2009).

45

In the present work of palaeomagnetism, we have selected prominently visible

20 dykes in and around the Greater Hyderabad and Ranga Reddy District and they are

shown in the Fig. 2.4

Fig. 2.4 Map showing the Dykes in the Study Area

46

REFERENCES

1. Acharyya. S.K., (2005). Geology and Tectonics of NE India. Jour. Geophys., V.

XXXVI, No. 1, pp. 35-49.

2. Aditi Mandal, Arijit Roy, Mayukhee Debnath, and Shankar Prasad Paul, (2012).

Petrology, geochemistry of hornblende gabbro and associated dolerite dyke of

Paharpur, Puruliya, West Bengal: implication for Petrogenetic Process and

tectonic setting. J. Earth Syst. Sci.121, No. 3, pp. 793-812.

3. Babuskin, V.D., (1972). Prognz Vodopritikov Ve gorni virabotki Vodozaboli,

Padyomlin Vod Ve Treshinovathde Zakarstovannin Paradan (Nedra) (in

Russian).

4. Balakrishna, S., (1964). Some studies on granites of Hyderabad from

Proceedings of the Seminar on Peninsular Geology, Geology Department,

Osmania University, Hyderabad, pp. 232-253.

5. Baleequddin Hussain, (1968). A note on the textural features of granites of

Hyderabad and their bearing on petrogeneisis. Bull. Geo. Soc. India. Vol. 5, pp.

8- 12.

6. Balk, R., (1937). Structural behavior of igneous rocks. Geol. Soc. Am. Mem. 5,

pp. 177.

7. Crawford, A. R. (1969). Reconnaissance Rb – Sr dating of the Precambrian

rocks of Southern Peninsular India, Jour. Geo. Soc. Ind., Vol. 10, No.2, pp. 117-

166.

8. Gnaneshwar. P., (1986). Lithogeochemical surveys in Hyderabad granites. Ph.D.

thesis Osmania University, p. 170.

9. Janardhan Rao, Y.J., (1965). The origin of Hyderabad granites. A new

interpretation. Jour. Ind. Geo. Sci. Assn. vol. 5, pp. 111-118.

47

10. Janardhan Rao, Y.J., and Sitaramaiah, S., (1968). The Pink alaskites of

Ghatkesar area, Hyderabad district, A.P. Jour. Osmania University (Science)

Golden Jubilee, Vol. pp. 75-98.

11. Kittu. N., (1967-68). Progress report for the field season (1966-67).

Investigation for molybdenite in parts of karimnagar district. A.P. G.S.I.

12. Kittu. N. and Krishnamurthy. A., (1966-67). Progress report for the field season

(1966-67). Investigation for molybdenite in parts of karimnagar district. A.P.

G.S.I.

13. Krishna, B., (2004). Geochemistry of Ground water and Soils of Jedimetla

Industrial Area, Ranga Reddy District, Andhra Pradesh, Ph.D. Thesis, Osmania

University, Hyderabad.

14. Mallikharjuna Rao. J., Poornachandra Rao G.V.S., and Sarma K.P., (2009).

Precambrian Mafic Magmatism of Shillong Plateau, Meghalaya and their

Evolutionary History. Journal of Geological Society of India, Vol. 73. pp. 143-

152.

15. Mazumber. S.K., (1986). The Precambrian framework of part of the Khasi Hills.

Meghalaya Ree. Geo. Sur. India, V. 117, pt.2, pp. 1-59.

16. Nivarani Devi and Sarma, K.P., (2006). Tectonostigraphic study of

conglomerates of Shillong Basin of Meghalaya. Jour. Geol. Soc. India, V. 68,

pp. 1100-1108.

17. Raghava Rao, M., (1974). Studies on the structure, petrology and engineering

properties of the rocks around Bhongir, Nalgonda district., A.P. Ph.D. thesis,

Osmania University.

18. Raja, N., (1959). Studies on granites of Hyderabad. Ph.D. thesis Osmania

University, Hyderabad.

19. Rao,Y.J., (1963). The Pink and Grey granites of Hyderabad and the problem of

their age relationship, Proc. Symp. On stratigraaphy. Age and correlation of the

Archaean Provinces of India.

48

20. Rao, Y.J., (1965). The origin of Hyderabad granites a new interpretation. Jour.

Ind. GeoSci. Assn., Vol.5, pp.111-118.

21. Satayanarana, B., (1983). Petrological studies and their bearing on the origin of

Hyderabad granites. Jour. Ind. Acad. Geoscience., Vol. 26. pp.1- 8.

22. Sitaramayya, S., (1968). Structure, Petrology and Geochemistry of Granites of

Ghatkesar, A.P., India, Ph.D. Thesis, Osmania .University, Hyderabad.